In a method of defining an animation of a virtual object, during which values for attributes of the virtual object are updated at each of a series of time points, a user specifies a structure representing the update that includes a plurality of items and one or more connections between respective items. Each item represents a respective operation. Each connection represents that data output by the operation represented by one item is input to the operation represented by the connected item. The user specifies that the structure comprises one or more items in a predetermined category associated with a predetermined process that may be executed at most a predetermined number of times at each time point. An item belongs to the predetermined category if performing the respective operation represented by that item requires execution of the predetermined process. One or more rules are applied.